Optical fibers are in widespread use today as transmission media because of their large bandwidth capabilities and small size. However, they are mechanically fragile, exhibiting low strain fracture under tensile loading and degraded light transmission when bent. Also, an optical fiber cable must be capable of withstanding tensile loads applied when the cable is routed along runs and bending stresses caused when the cable is pulled through turns. Accordingly, cable structures have been developed to protect mechanically the optical fibers thereby rendering them a realizeable transmission medium. Tensile strength for optical fiber cables has been provided by helically wrapped metallic wires for large fiber count cables and by high strength yarn such as Kevlar.RTM. or glass yarn, for example, for those having a relatively small fiber count.
As the use of optical fiber has grown, so too have the requirements which must be met for its use in special situations. In those special situations, tensile strength may or may not be a requirement, but other properties must be achieved.
For example, in the construction of many buildings, a finished ceiling, which is referred to as a drop ceiling, is spaced below a structural floor panel that is constructed of concrete. The space between the ceiling and the structural floor from which it is suspended serves as a return-air plenum for elements of heating and cooling systems as well as a convenient location for the installation of communications cables including those for computers and alarm systems. It is not uncommon for these plenums to be continuous throughout the length and width of each floor.
When a fire occurs in an area between a floor and a drop ceiling, it may be contained by walls and other building elements which enclose that area. However, if and when the fire reaches the plenum, and if flammable material occupies the plenum, the fire can spread quickly throughout an entire story of the building. The fire could travel along the length of cables which are installed in the plenum. Also, smoke can be conveyed through the plenum to adjacent areas and to other stories.
Generally, a non-plenum cable sheath system which encloses a core of insulated copper conductors and which comprises only a conventional plastic jacket does not possess acceptable flame spread and smoke evolution properties. As the temperature in such a cable rises, charring of the jacket material begins. Afterwards, conductor insulation inside the jacket begins to decompose and char. If the jacket char retains its integrity, it functions to insulate the core; if not, it is ruptured by the expanding insulation char, exposing the virgin interior of the jacket and insulation to elevated temperatures. The jacket and the insulation begin to pyrolize and emit flammable gases. These gases ignite and, because of air drafts within the plenum, burn beyond the area of flame impingement, propagating flame and evolving smoke.
The prior art has addressed the problem of cable jackets that contribute to flame spread and smoke evolution through the use of special materials, such as fluoropolymers, for example. These, together with enclosing layers of other materials, have been used to control char development, jacket integrity and air permeability to minimize restrictions on choices of materials for insulation within the core. One prior art small size plenum cable is disclosed in U.S. Pat. No. 4,284,842 which was issued on Aug. 18, 1981, in the names of C.J. Arroyo, et al. That cable includes a polyvinyl chloride insulated metallic conductor core which is enclosed in a corrugated metallic shield and helically wrapped tape comprised of Kapton.RTM. polyimide film.
The problem of acceptable plenum cable design is complicated somewhat by the trend to the extension of the use of optical fiber transmission media from a loop to building distribution systems. Not only must the optical fibers be protected from transmission degradation, but also they have properties which differ significantly from those of copper conductors and hence require special treatment. As mentioned earlier, transmitting optical fibers are mechanically fragile, exhibiting low strain fracture under tensile loading and degraded light transmission when bent. The degradation in transmission which results from bending is known as macrobending loss. Another mechanism which causes degradation of transmission is known as microbending loss. This loss can occur because of shrinkage during cooling of the jacket and because of differential thermal contractions when the thermal properties of the jacket material differ significantly from those of the enclosed optical fibers.
Cables for these special applications often must meet requirements established by industry watchdog organizations or government regulatory agencies. For example, in order to be acceptable for use in a plenum, a cable must meet the requirements of Underwriters Laboratories test 910.
Other uses of optical fibers for communications continue to surface. For example, it is expected that optical fiber wiring harnesses will be used in jet aircraft or in nuclear installations where they also will be subject to scrutiny of watchdog organizations or of government regulatory agencies.
For such use in aircraft, the optical fiber cable must not only have a specified tensile strength to enable pulling throughout the aircraft frame, but also it must have other properties. For use in jet aircraft, for example, the sought-after cable must be able to function for a relatively long period of time in temperatures in the range of 500.degree. F. Also, once a conflagration begins, aircraft operators require a specified time within which to make a decision as to a course of action. A candidate cable must pass a flame test in which it is exposed to a temperature of 2000.degree. F. for fifteen minutes. This latter test is used to evaluate the performance of the cable under catastrophic conditions. As should be apparent, the temperature requirements for jet aircraft use exceed those for plenum cable.
Optical fiber transmission is affected moreso than metallic conductors by high temperature levels. In copper cables, although the plastic insulation may burn, the copper conductors will continue to be operative for a specified time. At the temperature levels expected in jet aircraft, the jacket and/or coating material which encloses the optical fiber will degrade. Inasmuch as the optical fiber itself cannot withstand the expected tensile loading, microcracks will develop unless the optical fiber is provided with suitable protection.
Nevertheless, there is great impetus for using optical fiber instead of copper conductors in presently manufactured aircraft. This desire is driven by the advantages which optical fiber enjoys over copper conductors. For example, optical fiber conductors are immune from electromagnetic interference, have a higher bandwidth than metallic conductors and are of a smaller size and lower weight while being more reliable.
Additionally, to qualify for use in jet aircraft, the sought-after cable must be sealed hermetically. This is required in order to prevent the ingress of contaminants such as jet fuel, methyl ethyl ketone, salt water and hydraulic fluid, at least some of which may be at elevated temperatures.
Presently, cables for use in jet aircraft and other high temperature applications include copper conductors insulated and or jacketed with high temperature resistant plastic materials such as Teflon plastic, for example. In those cables, there is not the same level of concern about the copper conductors as there is about the optical fibers in an optical fiber cable used in the same environment. Should the plastic be dissolved by contaminating fluids, the metallic conductors most likely continue to provide for transmission. To the contrary, in an optical fiber cable, once the protective jacket and coating are removed from the cable, the optical fibers are susceptible to attack and transmission loss ensues.
Furthermore, the sought-after cable must be flexible so that it can be routed along tortuous paths in jet aircraft or in other confined spaces. Of course, as is well known, cables of the prior art have included corrugated shields which have provided flexibility, particularly for large pair count metallic conductor cables.
One need not dwell too long to arrive at other uses for hermetically protected, high temperature resistant cables. What is sought is a sheath protection system which may be used for a number of different cables. It must be one which is hermetically sealed, which will withstand a grossly elevated temperature for a predetermined time and which will be operative for a predetermined time should a catastrophe occur. The sought-after cable must have a relatively small diameter and requisite tensile strength. Also, it must be flexible and be relatively inexpensive to manufacture. Seemingly, the prior art does not include a cable which possesses all the aforementioned properties.